[PPC64] Fix trapstk overflow

In some scenarios, the 4K trapstk may overflow, corrupting tmpstk.

This was observed during remote debugging, with the following steps:

At remote host (R):

• enter kdb during boot
• switch to gdb backend

At local host (L):

• attach gdb to R
• try to read an invalid memory position

At R:

• a DSI trap occurs and kdb restarts (all this occurs on trapstk)
• while printing the stacktrace, trapstk overflows and corrupts tmpstk

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22200